The present invention relates generally to optical focusing systems and, more specifically, to an improved dual beam rangefinder suitable for use in a single lens reflex camera.
Various types of rangefinders or focusing system have been proposed and are in use in single lens reflex cameras. One such system which has met with some success is the dual beam rangefinder disclosed in U.S. Pat. No. 3,696,724 issued to Hartmann. This dual beam rangefinder includes two reflecting means that are mounted between the zoom or afocal lens portion and the prime lens portion of an objective lens so as to reflect lightrays from the scene to be photographed into the viewfinder of the camera. The reflecting means are offset from the optical axis of the camera and project separate reflected images which are received by an optical system forming a part of the viewfinder. When the images from the reflecting means are superimposed on the focal plane of the viewfinder, the lens system of the camera is in focus. When the images are not superimposed, the lens system is out-of-focus and must be changed.
While such dual beam rangefinders provide several advantages over other prior art focusing systems, they have not been entirely satisfactory. For example, because the reflecting means are displaced from the optical axis of the lens, each of the images received at the plane of the viewfinder is slightly distorted. Moreover, since the reflecting means are positioned in spaced relationship and at generally opposed peripheral areas of the optical path, the images are distorted in opposite directions. Similarly, due to the inability to mount the reflecting means in exact parallel alignment, slight rotation of each reflected image occurs, relative to the other, which cannot be compensated through adjustment of the lens or viewfinder system. As a result of the distortion and rotation of the reflected images it is impossible to superimpose the two images over the entire field of the viewfinder focal plane. Thus, even when the objective lens of the camera is in focus on the object to be photographed, the two reflected images are not in complete superimposition at the plane in the viewfinder. Actually, when the objective lens are focused on the subject, the central portions of the reflected images will appear properly superimposed, or in focus, at the plane of the viewfinder, but the distortion and rotational effects become more apparent further from the optical axis of the viewfinder such that the peripheral areas of the reflected images will appear to be out of focus. Of course, the inability to obtain complete superimposition of the two reflected images over the full field of the viewfinder plane is undesirable in that it may adversely affect rangefinder accuracy and viewfinder clarity. In addition, the non-focused peripheral areas of the viewfinder images may prove irritating to the user of the camera.